The present invention relates to a photostable cosmetic composition for the protection of skin against UV radiation comprising in combination a UV-A filter and a substituted dialkylbenzalmalonate. It also relates to the use of this composition in protecting skin against UV radiation, to a method of stabilizing a UV-A filter by the substituted dialkylbenzalmalonate, to the use of substituted dialkylbenzalmalonates as broad-band solar filters and to novel substituted dialkylbenzalmalonates.
Light at wavelengths between 280 nm and 400 nm is known to brown human skin and light at wavelengths between 280 and 320 nm, known as UV-B, produce erythemas and cutaneous burns which may hinder tan development; the UV-B radiation must therefore be filtered out.
It is also known that UV-A radiation at wavelengths between 320 and 400 nm and producing browning of the skin can damage the skin, especially when it is sensitive or continually exposed to the sun's rays. UV-A radiation produces, in particular, loss of skin elasticity and the appearance of wrinkles leading to premature ageing. It favors the start of the erythematous reaction or, in some subjects, amplifies the same and may even trigger phototoxic or photoallergic reactions. Thus it is also desirable to filter out UV-A radiation.
French patents FR 2 326 405 and FR 2 440 933 and European patent EP 114 607 describe dibenzoylmethane derivatives as UV-A filters. The patents propose combining these UV-A filters with various UV-B filters to absorb all UV radiation at wavelengths between 280 and 380 nm. Among the UV-B filters used in combination with these dibenzoylmethane derivatives are paramethoxycinnamic acid esters, in particular 2-ethylhexyl paramethoxycinnamate, sold by GIVAUDAN under the trade name "PARSOL MCX".
Unfortunately, when used in combination with this UV-B filter, the dibenzoylmethane derivatives described in the aforementioned patents are not sufficiently photochemically stable to be able to provide constant protection during prolonged exposure to the sun. This necessitates repeated applications at regular and frequent intervals in order to obtain effective skin protection against UV radiation.